Successful fabrication of multilayer printed circuit boards requires bonding together of copper and resin layers. However, direct bonding of copper and resin layers does not provide sufficient bonding strength. Therefore, it is common to improve copper-resin bonding strength by depositing on the copper surface an oxide layer, such as cuprous oxide, cupric oxide, or the like. Formation of the oxide layer, which turns the pink copper surface a black-brown color, creates minute unevennesses on the copper surface which provide an interlocking effect between the copper surface and resin, thus improving bonding strength.
However, copper oxides are readily hydrolyzed and dissolved upon contact with acid. Because various acid treatments are used in later stages of fabrication of multilayer circuit boards, oxide layer deposition has been problematic at best. Acid attack on the oxide layer is commonly referred to in the industry as "pink ring", because as acid strips the black-brown oxide layer from the surface, a ring of bare pink copper becomes evident.
The problem of vulnerability of the oxide layer to acid was solved by the method described in U.S. Pat. No. 4,642,161 to Akahoshi et al., herein incorporated by reference in its entirety; the Akahoshi et al. patent has been assigned to Hitachi, Ltd. The Akahoshi et al. method is also described in Akahoshi et al., Circuit World 14(1) (1987), and in the Hitachi, Ltd. technical publication "The Chemical Reduction Treatment of Copper Oxide, DMAB Method (Technology for the Elimination of Pink Ring", both of which references are herein incorporated by reference in their entireties.
In the Akahoshi et al. method, the copper oxide layer is reduced to metallic copper by means of a reducing solution containing an amine borane compound as the active reducing agent. The minute unevennesses created on the copper surface from oxidation remain following reduction, so that the metallic copper surface produced as a result of the reduction process will form a sufficiently strong bond with a resin. In contrast to cupric oxide and cuprous oxide, which are both soluble in acid, the metallic copper surface resulting from the reduction process, which is the same black-brown color as the oxide layer, has good acid resistance. Therefore by reducing the copper oxide to metallic copper, the acid resistance of the surface or panel is increased, and there is a reduced likelihood of the appearance of "pink ring".
The presently known reducing agents which are capable of reducing cupric oxide to metallic copper are amine boranes represented by the general formula: BH.sub.3 NHRR' (wherein R and R' are each a member selected from the group consisting of H, CH.sub.3, and CH.sub.2 CH.sub.3), such as dimethylamine borane (DMAB) and ammonia borane. These reducing agents are quite expensive.
During the reduction process the reducing agents are consumed by the copper oxide on the panels. The stoichiometric consumption of reducing agent can be determined from the half reaction: EQU BH.sub.3 +3H.sub.2 O.fwdarw.H.sub.3 BO.sub.3 +6H.sup.+ +6e.sup.-
It is known that the reducing agent continues to be consumed even after all of the cupric oxide on the panels has been reduced to copper metal, and no additional cupric oxide is introduced into the solution. Consumption of reducing agent continues after reduction of the copper oxide panels because, it is theorized, that the reduced copper oxide from the panels is still present in the reducing solution, and may be reoxidized or may catalyze the hydrolysis of the DMAB. Therefore, the reducing agents are consumed in greater than the stoichiometric amount necessary to reduce the copper oxide on the panels. The excessive consumption of the reducing agent shortens the usable lifetime of the reducing solution, and ultimately results in higher operating costs for the process.
Thus, it is clearly desirable to develop a method and composition to stabilize the reduction process so as to reduce consumption of the costly reducing agent, while ensuring that the metallic copper layer resulting from such a stabilized reduction process has good bonding properties and acid resistance.